Grout formulation for disposal of low-level and hazardous waste streams containing fluoride

ABSTRACT

A composition and related process for disposal of hazardous waste streams containing fluoride in cement-based materials is disclosed. The presence of fluoride in waste materials acts as a set retarder and as a result, prevents cement-based grouts from setting. This problem is overcome by the present invention wherein calcium hydroxide is incorporated into the dry-solid portion of the grout mix. The calcium hydroxide renders the fluoride insoluble, allowing the grout to set up and immobilize all hazardous constituents of concern.

FIELD OF THE INVENTION

The present invention relates to a method and composition for disposingof low-level and hazardous waste streams containing fluoride incement-based materials. More particularly, the present invention relatesto the incorporation of calcium hydroxide into grout formulations toinactivate fluorides present in hazardous and low-level waste streams.

BACKGROUND OF THE INVENTION

Fixation or immobilization of wastes in cement-based materials, commonlyreferred to as grouts, is an important waste management method.Formulations prepared by mixing the waste material with a dry-solidblend consisting of cement, fly ash and clays are commonly referred toas grouts. Grout properties of importance in waste immobilizationvariously include the rheologic properties of freshly mixed grouts, thestructural stength of cured grouts, the leach properties of cured groutsand the amount of grout phase separation exhibited at various timesduring curing.

A problem which is encountered in waste disposal is the presence offluorides in the waste materials. Fluoride acts as a set retarder forcement based grouts and therefore presents a serious problem forimmobilization of waste containing fluoride.

Hydrofracture grouts are composed primarily of cementitious phases.Therefore, it is important to understand the reactions that occur duringthe hydration cement. These reactions are discussed in detail inStinton, D. P., et al, Characterization of Hydrofracture Grouts forRadionuclide Migration, Oakridge National Laboratory, July 1983. Thisarticle shows that a significant amount of calcium hydroxide isgenerated as a result of the hydration of cementitious phases present ingrout materials. In addition, this article also teaches that thiscalcium hydroxide, sometimes called Portlandite, apparently reacts withcarbon dioxide from the air to form carbonates. This article does notaddress the problem created by fluoride in the waste material to beincorporated in the grout. Moreover, the amount of calcium hydroxidegenerated by hydration of the cementitious phases does not suffice toinactivate undesirable fluoride present in the waste material and, as aresult, fluorides retard or prevent setting of the grout.

Struxness, E. G., et al, Engineering Development of Hydraulic Fracturingas a Method for Permanent Disposal of Radioactive Wastes, OakridgeNational Laboratory, Aug. 9, 1968, discusses the initial development offormulations used as hydrofracture grouts. The primary problem addressedby this article is the leaching of radioactive strontium from the groutmaterial. This article teaches that a significant amount of calcium isleached out of the grout material in addition to the salts normallypresent in the waste material. Presumably, this calcium came for solublecompounds formed in the cement as it set. The analytical data confirmedthat the amount of radioactive strontium leached out of the groutmaterial was directly proportional to the amount of calcium whichleached from the grout. Accordingly, the article teaches that reductionof the amount of leachable calcium in the grout material is desirablesince it will reduce the amount of radioactive strontium leached fromthe grout.

Additional research has been performed to develop the requiredcementitious grouts for disposal of low-level radioactive waste. Theresults of this research are embodied in the following articles: Moore,J. G., et al, Strontium Leachability of Hydrofracture Grouts forSludge-Slurries, Oakridge National Laboratory, March 1982; Moore, J. G.,Development of Cementitious Grouts for the Incorporation of RadioactiveWastes. Part I: Leach Studies, Oakridge National Laboratory, April 1975;and Moore, J. G., Development of Cementitious Grouts for theIncorporation of Radioactive Wastes. Part II: Continuation of Cesium andStrontium Leach Studies, Oakridge National Laboratory, September 1976.These articles deal with the adaptation of cementitious grouts forpermanent immobilization of low-level radioactive wastes containingstrontium and cesium. These reports recognize that reductions in theamount of cement in the grouts leads to a corresponding reduction in theamount of strontium which leaches out of the grout materials. Thisphenomenon is directly attributable to the reduction in the amount ofleachable calcium present in the grout material as a result of thereduction in the amount of cement present in the material. Thesearticles also found that the amount of strontium leaching out of thegrout material could be reduced by addition of stable strontium incombination with calcium ions in the form of calcium chlorides.

Tallent, O. K. et al, Fixation of Waste Materials in Grouts. Part I:Empirical Correlations of Formulation Data, Oakridge NationalLaboratory, March 1986, explores the properties of cementitiousmaterials used for disposal of waste materials. This publicationaddresses the variations in critical properties which occur as a resultof variations in the composition of the dry-solid materials used toformulate the grout.

SUMMARY OF THE INVENTION

The present invention relates to a composition which is used toformulate a waste disposal grout that inactivates fluoride ions in thewaste comprising from about 30 to about 70 weight percent cement; fromabout 10 to about 60 weight percent fly ash; from about 3 to about 13weight percent clay; and from about 5 to about 20 weight percent calciumhydroxide.

In a second embodiment the present invention relates to a method for theimmobilization of fluoride ion containing waste in cement basedmaterials comprising the steps of mixing aqueous waste materialcontaining fluoride ions with from about 6.0 to about 10.0 lbs of adry-solid material per gallon of said waste material. The dry-solidmaterial comprises from about 30 to about 70 weight percent cement; fromabout 10 to about 60 weight percent fly ash; from about 3 to about 13weight percent clay; and from about 5 to about 20 weight percent calciumhydroxide. Finally, the mixture is cured to produce a solid cement-basedmaterial.

Accordingly, it is the primary object of the present invention toprovide a method and composition for inactivating fluorides present inhazardous waste materials so that cement based grout can be used toimmobilize such waste materials.

Another object of the present invention is to provide a dry-solidmaterial for use in formulation of cement based grout that canimmobilize fluoride-containing waste materials.

These and other objects of the present invention will be apparent to oneof ordinary skill in the art from the detailed description whichfollows.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

The invention relates to a method and composition of matter whereincalcium hydroxide is added to cement-based waste disposal groutcontaining fluoride to render the fluoride insoluble. This allows thegrout to set up to proper structural integrity for immobilization of allhazardous constituents of concern. This also substantially increases therate of hydration of the cement thereby eliminating drainable water andproducing an end product which exhibits properties favorable todelisting waste. The invention is applicable to the disposal of anyaqueous waste material containing fluorides in cement-based grouts. Suchwaste materials include nuclear reactor cladding removal waste and wastegenerated by electroplating reactions.

In general, grout materials are formulated by preparing a dry-solidblend of cement, fly ash and clays and mixing this dry-solid blend withan aqueous waster material to obtain a cement-based material commonlyreferred to as grout. The methods of preparation of grout materials arediscussed in more detail in Tallent, O. K. et al, Fixation of WasteMaterials in Grouts. Part I: Empirical Correlations of Formulation Data,Oakridge National Laboratory, March 1986, which is hereby incorporatedby reference. The grout materials are made by mixing the aqueous wastestream with the dry-solid blend in the manner set forth by the AmericanSociety for Testing and Materials in 1984 Annual Book of ASTM Standards,Volume 04.02, Publication C192-81, "Standard Method of Making and CuringConcrete Test Specimens in the Laboratory", and Volume 04.01,Publication C305-82, "Standard Method for Mechanical Mixing of HydraulicCement Pastes and Waters of Plastic Consistency", which are herebyincorporated by reference.

Aqueous waste streams containing fluoride ions may be disposed of inaccordance with the present invention. The waste stream may also containany of a number of other industrial waste materials such as strontium,cesium, nitrates, sulfates, hydroxides and other materials such asmetals. One such waste stream requiring disposal is cladding-removalwaste from nuclear reactors. This waste stream is generated duringremoval of the protective cladding on spent nuclear fuel rods.

The dry-solid compositions used to formulate grout materials inaccordance with the present invention include cement, fly ash, clay andcalcium hydroxide. The calcium hydroxide in the dry-solid blendneutralizes fluoride ions in the waste stream thereby allowing the groutto set. Other materials may be incorporated into the dry-solid materialsof the present invention as well. The incorporation of additionalmaterials into the dry-solid mixture is dictated by the specificapplication to which the grout formulation is directed. The presence ofcertain unusual compounds in aqueous waste material may necessitate theincorporation of additional compounds into the dry-solid material.

The dry-solid mixture generally includes from about 30 to about 70weight percent cement, and more preferably includes from about 50 toabout 70 weight percent cement. Various cements are useful in theinvention. The most preferred cements are the Portland-type cements suchas Type I Portland cement, Type II Portland cement and Type III Portlandcement. As appreciated by those of ordinary skill in the art, theparticular type of Portland cement to be used in the dry-solid mixtureis selected in order to obtain specific properties in the groutmaterials.

The dry-solid mixture also contains from about 10 to about 60 weightpercent fly ash, and more preferably from about 15 to about 40 weightpercent fly ash. The fly ash is incorporated into the grout material asa filler to reduce the amount of cement in the grout. One reason forthis substitution is that fly ash is significantly less expensive thancement. Moreover, as the amount of fly ash in the dry-solid formulationis increased, there is a corresponding increase in the compressivestrength of the resulting grout material. A high compressive strength isa desirable property of grout material since it will make the grout moreresistant to compressive forces and thereby reduce the amount ofmaterial which leaches out of the grout as a result of compressiveforces. The preferred type of fly ash to be incorporated in the grout ofthe invention is Centralia, Wash. Class F fly ash. Other types of flyash may also be used depending on their cost and availability.

The dry-solid mixture of the invention also includes from about 3 toabout 13 weight percent clay, and more preferably from about 6 to about10 weight percent clay. The clay is present in the dry-solid materialprimarily for the purpose of providing an ion-exchange medium. Thepreferred clay for use with the invention is illitic clay which has thegeneral formula (OH)₄ K_(x) (Al₄.Fe₄.Mg₄. Mg₆)Si_(8-x).Al_(x))O₂₀. Otherion exchange clays may also be used in the invention.

Finally, the dry-solid material of the invention includes from about 5to about 20 weight percent of calcium hydroxide, and more preferablyfrom about 10 to about 14 weight percent calcium hydroxide. The calciumhydroxide is incorporated in the grout material in order to react withfluoride ions present in the waste material and produce insolublecalcium fluoride. The reaction between fluoride and calcium hydroxidetakes place in the mixer and generates calcium fluoride and hydroxideions. The calcium hydroxide is preferably added to the dry-solid blendas hydrated lime due to the inexpensive nature of this form of calciumhydroxide. However, other forms of calcium hydroxide such as reagentgrade calcium hydroxide may be employed.

Attapulgite-150 clay, because of its apparent interference with thecalcium hydroxide-fluoride reaction, is preferably not included in thisgrout formulation. However, it may be included in the grout formulationswithout seriously affecting the calcium hydroxide neutralization offluoride ions. Attapulgite-150 clay is normally added to the dry-solidblend used in grout formulation to reduce the amount of drainable waterwhich is generated by curing the grout material. Despite the absence ofAttapulgite-150 clay from the preferred formulation, the grout producedby this method meets the criteria for drainable water and far exceedsthe criteria for compressive strength of grouts. It also exhibits a verylow viscosity, demonstrating good fluidity necessary for pumping thegrout into the ground.

Fluoride-containing aqueous waste materials may be disposed of inaccordance with the present invention. One such aqueous waste materialis generated during removal of the protective cladding on spent nuclearfuel rods. Cladding-removal waste, which contains fluoride, cesium,ammonia and zirconium, must be chemically pretreated by the addition ofan aqueous caustic solution, such as sodium hydroxide, before itsimmobilization in a cement based grout. The resulting slurry is furthertreated by adjusting the pH and the addition of sodium nitrite to meetstorage tank specifications. The solids are allowed to settle and thesupernate liquid is decanted for ultimate disposal as a separate wastestream. The resulting sludge, designated neutralized cladding-removalwaste, becomes the waste feed stream to be immobilized in accordancewith the present invention. The precipitation of zirconium by eitherdirect strike or co-strike neutralization is a well establishedprocedure. However, the neutralized cladding-removal waste differs fromwastes previously fixed in grout in that it contains significant amountsof fluoride ions. The fluoride ions strongly retard the setting ofcement-based grouts and thus can prevent the grout from obtaining anacceptable compressive strength and can cause excessive bleed water.

The neutralized cladding-removal waste is mixed with from about 6.0 toabout 10.0 pounds of the dry-solid blend of the present invention pergallon of waste material. The mixing is accomplished in accordance withthe procedures detailed by the American Society for Testing andMaterials in 1984 Annual Book of ASTM Standards, Volume 04.02,Publication C192-81, "Standard Method of Making and Curing Concrete TestSpecimens in the Laboratory", and Volume 04.01, Publication C305-82,"Standard Method for Mechanical Mixing of Hydraulic Cement Pastes andWaters of Plastic Consistency". After mixing, the grout is injected intothe ground while it is still in a flowable condition. It is then allowedto cure in the ground and remain there.

The following examples are provided to illustrate the present invention.

EXAMPLE 1

                  TABLE 1                                                         ______________________________________                                        Dry-Solid Blend for Disposal of Neutralized Cladding Removal                  Waste (NCRW) with Calcium Hydroxide                                           Material                wt %                                                  ______________________________________                                        Type III Portland Cement                                                                              42.0                                                  ASTM Class F Centralia, WA, fly ash                                                                   42.0                                                  Attapulgite-150 clay    5.0                                                   Calcium Hydroxide       10.0                                                  Barium Hydroxide        1.0                                                   ______________________________________                                    

The grouts made with the above dry-solid blend exhibited the followingproperties. All NCRW sludge includes fluorides.

                  TABLE 2                                                         ______________________________________                                        Parameters for grouts made with NCRW sludge mixed in the ratio                of 6 lb/gal with dry-solids, (assumes no radioactive cesium).                            Waste dilution (%)                                                 Parameter    20     33        50      70                                      ______________________________________                                        Apparent     93     52        30      21                                      viscosity (cP)                                                                10 M         18     12        8       89                                      gel strength                                                                  (lb.sub.f /100 ft.sup.2)                                                      Fluid consistency                                                                          0.06   0.02      0.01    0.02                                    index, K'                                                                     (lb.sub.f · sec.sup.n' /ft.sup.2)                                    Flow behavior                                                                              0.45   0.52      0.56    0.42                                    index, n'                                                                     28 days drainable                                                                          4.48   6.16      0.0     0.0                                     water (vol %)                                                                 Density (lb/gal)                                                                           12.81  12.59     12.28   11.90                                   28 days compressive                                                                        S      23 ± 40                                                                              346 ± 2                                                                            300                                     strength (psi)                                                                At reference                                                                  conditions:                                                                   Critical velocity                                                                          86     58        39      37                                      (gal/min)                                                                     Frictional   11.2   6.0       3.4     2.8                                     pressure loss                                                                 per 100 ft (psi)                                                              ______________________________________                                         S -- Material did not setup, no test performed                                Waste dilution % corresponds to the % water by volume in the waste            formulation.                                                             

EXAMPLE 2

                  TABLE 3                                                         ______________________________________                                        Dry-Solid Blend for Disposal of Neutralized Cladding                          Removal Waste (NCRW) with Lime                                                Material          Amount (wt %)                                               ______________________________________                                        Portland cement, Type III                                                                       57                                                          fly ash, class F, 25                                                          (Centralia, WA)                                                               Indian Red pottery clay                                                                          8                                                          Lime [Ca(OH).sub.2 ]                                                                            10                                                          ______________________________________                                    

The grouts made with the above dry-solid blend exhibited the followingproperties. All NCRW sludge includes fluorides.

                  TABLE 4                                                         ______________________________________                                        Effect of dry-solids blend variations on grout properties.                    Mix ratio (lb/gal)                                                                           5      6       7      8                                        ______________________________________                                        Apparant viscosity (cP)                                                                      14     21      34     55                                       10 min gel strength                                                                          3      5       17     >300                                     (lb.sub.f /100 ft.sup.2)                                                      Density (lb/gal)                                                                             12.10  12.48   12.80  13.00                                    Fluid consistency index                                                                      0.001  0.003   0.01   0.02                                     (K') [lb.sub.f · sec.sup.n' /ft.sup.2 ]                              Flow behavior index (n')                                                                     0.84   0.70    0.59   0.53                                     28 day phase separation                                                                      24.92  17.84   0.0    0.0                                      (vol %)                                                                       24 hour penetration                                                                          0.0    0.0     280    530                                      resistance (psi)                                                              7 day penetration                                                                            0.0    0.0     240    920                                      resistance (psi)                                                              28 day penetration                                                                           0.0    0.0     2840   4800                                     resistance (psi)                                                              28 day compressive                                                                           S      S       629 ± 14                                                                          811 ± 18                              strength (psi)                                                                At reference                                                                  conditions:                                                                   Reynolds number                                                                              7430   4988    2726   1684                                     Frictional pressure                                                                          1.3    2.0     3.8    6.3                                      loss per 100 ft                                                               of pipe (psi)                                                                 Critical velocity                                                                            17     26      42     58                                       (gal/min)                                                                     Pump head pressure                                                                           0.5    0.8     2.8    >50.0                                    (psi)                                                                         ______________________________________                                         S -- material did not setup, no test performed.                          

From this data it is apparent that grouts formulated with lime and noAttapulgite-150 clay exhibit the properties necessary for waste disposalin cement based materials.

It will be apparent to those of ordinary skill in the art that variousmodifications and variations of the invention can be made withoutdeparting from the scope or spirit of the invention. Accordingly, thescope of the invention shall be determined by the claims appendedhereto.

What is claimed is:
 1. A composition which may be used to formulate awaste disposal grout for immobilization of fluoride ion-containingaqueous waste materials comprising:from about 30 to about 70 weightpercent cement; from about 10 to about 60 weight percent fly ash; fromabout 3 to about 13 weight percent clay; and from about 5 to about 20weight percent calcium hydroxide.
 2. A composition in accordance withclaim 1 wherein said clay comprises illitic clay.
 3. A composition inaccordance with claim 1 wherein said composition comprises:from about 50to about 70 weight percent cement; from about 15 to about 40 weightpercent fly ash; from about 6 to about 10 weight percent illitic clay;and from about 10 to about 14 weight percent calcium hydroxide.
 4. Acomposition in accordance with claim 1 wherein said calcium hydroxidecomprises hydrated lime.
 5. A composition in accordance with claim 4wherein said composition is a dry-solid blend.
 6. A method for theimmobilization of fluoride ion containing waste in cement-basedmaterials comprising the steps of:mixing an aqueous waste materialcontaining fluoride ions with from about 6.0 to about 10.0 pounds of adry-solid material per gallon of said waste material, said dry-solidmaterial comprising from about 30 to about 70 weight percent cement,from about 10 to about 60 weight percent fly ash, from about 3 to about13 weight percent clay, and from about 5 to about 20 weight percentcalcium hydroxide; and curing said mixture to produce a solidcement-based material.
 7. A method in accordance with claim 6 whereinsaid clay comprises illitic clay.
 8. A method in accordance with claim 7wherein said dry solid material comprises:from about 50 to about 70weight percent cement; from about 15 to about 40 weight percent fly ash;from about 6 to about 10 weight percent illitic clay; and from about 10to about 14 weight percent calcium hydroxide.
 9. A method in accordancewith claim 8 wherein said calcium hydroxide comprises hydrated lime.